The invention relates to systems and methods for maintaining an active element at a selected bias point, and more particularly, to systems and methods that passively compensate moving coil transducers to compensate for pressure fluctuations.
A moving coil transducer for underwater applications is similar to a loudspeaker in that it is designed with a very soft suspension system to provide a low natural resonant frequency. Because of this soft, structurally compliant suspension system, a pressure compensation system is required to keep the forces acting on the moving radiating piston in static equilibrium. By equalizing the interior pressure to the exterior pressure, the radiating piston will maintain its neutral position. It is essential that a neutral position for the radiating piston be maintained because of mechanical limitations associated with alignment of the radiating piston with the magnetic driver. Large excursions from a neutral position will cause the piston to exceed the boundary of the applied magnetic field with an associated reduction in output power and an increase in distortion. A typical maximum pressure imbalance of only .15 psi acting on the radiating piston is allowed for these types of transducers. Pressure equalization must be maintained as the exterior hydrostatic pressure is both increased and decreased.
Until now, these types of transducers have used pressurized gas in back of the radiating piston to equalize the interior transducer pressure to the exterior hydrostatic pressure acting on the front of the radiating piston. For applications at shallow depths, this can be accomplished easily by using a gas filled bladder. As the hydrostatic pressure increases, the bladder compresses under the hydrostatic load. The compressed bladder decreases the interior volume of gas. As the volume of gas decreases, the pressure increases. Pressure equilibrium is obtained when the bladder has compressed sufficiently such that the interior pressure is equal to the exterior pressure. This method is called a xe2x80x9cPassive Gas Compensation System.xe2x80x9d It is not practical for applications at deeper depths because the size of the bladder becomes prohibitively large.
Traditionally, for deeper depths, this type of transducer has employed a different method of gas compensation which injects high pressure gas into the interior of the transducer. This method of gas compensation is referred to as an xe2x80x9cActive Gas Compensation System.xe2x80x9d This type of compensation system is very complex because it requires a method to sense the interior and exterior pressures and control the addition of high pressure gas to the interior of the transducer and the exhaust of this gas from the transducer. This method of gas compensation also requires that high pressure gas be carried as part of the transducer system. High pressure gas containment and associated plumbing is a safety hazard. In addition, because the gas is exhausted and not recovered, the mission life for this type of system is very limited. This is particulary troublesome for systems such as the Mk30 Mod 2 Target Undersea Vehicle (TUV) system that has only a small allocated volume for transducer components. Consequently, this type of gas compensation system would greatly limit the mission life of the Mk30 Mod 2 TUV system. The vehicle would have to be brought to the surface frequently and the high pressure gas replenished. Another disadvantage of this type of system is that, although the gas provides pressure equalization for the radiating piston, the compliance of the gas decreases as the inverse square of the absolute pressure of the gas. Hence, as the system changes operating depth, the compliance of the suspension systems changes and the resonant frequency of the system will change. As the transducer is lowered to greater depths, the resonant frequency will increase considerably as the compliance of the gas behind the piston decreases. The hazards of handling high pressure gas containment systems, the requirements for replenishment of the gas supply, and the changing performance of the transducer make the Active Gas Compensation System a very unattractive, unreliable compensation system.
The systems and methods described herein can equalize the pressure between an interior cavity and an environment exterior to the interior cavity, while providing for acoustic isolation between the two environments so that acoustic energy propagating through one environment does not cause acoustic vibrations in the other environment. In one application, these pressure compensation systems are employed to equalize the pressure on either side of a moving coil projector, thereby reducing the deleterious effect that a pressure differential across the moving coil projector can have on the operation of the moving coil projector and reducing the likelihood that propagating acoustic energy can result in phase cancellation that reduces the acoustic performance of the moving coil projector.
In one embodiment, the systems include pressure compensation devices for use with a transducer assembly that has a moving coil and a diaphragm. The pressure compensation device can include a housing having an interior cavity capable of being filled with fluid and dimensioned for receiving and enclosing the moving coil of the transducer assembly. A resilient bladder can be disposed within the housing and can have a first portion in communication with an operating environment and a second portion in communication with a fluid reservoir maintained within the housing. An acoustic filter can couple to the fluid reservoir and attenuate acoustic energy propagating at selected frequencies within the fluid reservoir, and a fluid passage can extend between the fluid reservoir and the interior cavity, whereby a pressure change in the operating environment acts on the resilient bladder and is communicated through the fluid reservoir and the fluid passage to adjust the pressure within the interior cavity.
In a further embodiment, the apparatus can include a compressible body disposed within the interior cavity. The compressible body can compress or expand in response to the movement of the projector within the cavity. The compressible body can be a slotted cylinder, a spring assembly, such as a belleville spring assembly or any other device capable of performing as a spring. In one embodiment the compressible body is an air filled compliant disk assembly capable of being compressed in response to a change in pressure within the interior cavity. The compliant disk assembly can also comprise a plurality of bladders filled with a compressible gas capable of being compressed in response to a change in pressure within the interior cavity.
In one embodiment, the filter can comprise a conduit coupled between the fluid reservoir and the interior cavity and having an interior passage for forming the fluid passage extending therebetween, and being dimensioned for resisting transmission of acoustic energy at selected frequencies between the interior cavity and the exterior environment.
The fluid passage can include a conduit coupled between the fluid reservoir and the interior cavity and dimensioned to allow fluid to pass at a rate selected as a function of the rate of pressure change of the operating environment.
The housing can comprise a body having a mass selected to resist vibration at selected frequencies, as well as a support rim for mounting to the transducer assembly.
The housing can also include a mounting rim for allowing the housing to be removeably and replaceably mounted to a surface.
In another aspect, the systems described herein include a modular moving coil transducer having pressure compensation for adjusting to pressure changes in an operating environment. The transducer can include a transducer assembly having a moving coil and a diaphragm, a housing having a fluid-filled interior cavity enclosing the moving coil to the transducer assembly, and having a resilient bladder disposed between the operating environment and a fluid reservoir maintained within the housing and being capable of deforming in response to a pressure change in the operating environment, a filter coupled to the fluid reservoir and capable of attenuating acoustic energy propagating at selected frequencies within the fluid reservoir, and a fluid passage extending between the fluid reservoir and the interior cavity, whereby a pressure change in the operating environment acts on the resilient bladder for being communicated through the fluid reservoir and the fluid passage to adjust the pressure within the interior cavity.
The transducer can also include a compressible body disposed within the interior cavity, as well as a complaint disk assembly capable of being compressed in response to a change in pressure within the interior cavity. The filter can comprise a conduit coupled between the fluid reservoir and the interior cavity and having an interior passage for forming the fluid passage, and being dimensioned for resisting transmission of acoustic energy at selected frequencies between the fluid reservoir and the interior cavity. The fluid passage includes a conduit coupled between the fluid reservoir and the interior cavity and dimensioned to allow fluid to pass at a rate selected as a function of the rate of pressure change of the operating environment.
The systems can also include target underwater vehicles capable of ascending and descending to different depths within a fluid environment, comprising a submersible body having a sidewall with a port for receiving a transducer assembly, and a modular transducer assembly mounted within the port, and having a moving coil projector including a moving coil and a diaphragm, a housing having a fluid-filled interior cavity enclosing the moving coil, and having a resilient bladder disposed between the fluid environment and a fluid reservoir maintained within the housing, a filter coupled to the fluid reservoir and capable of attenuating acoustic energy propagating at selected frequencies within the fluid reservoir, and a fluid passage extending between the fluid reservoir and the fluid-filed interior cavity, whereby a pressure change arising from a change in depth within the fluid environment acts on the resilient bladder to communicate the pressure change through the fluid reservoir and the fluid passage to adjust the pressure within the interior cavity.
Other objects of the invention will, in part, be obvious, and, in part, be shown from the following description of the systems and methods shown herein.